Multicarrier modulation (MCM) is based on the idea of splitting a high-rate wideband signal into multiple lower-rate signals, where each signal occupies a narrower band. Orthogonal frequency division multiplexing (OFDM) has proved itself as one of the most popular MCM techniques and is currently used in many wireless communication systems such as 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), 802.11, etc. OFDM offers many advantages such as robustness to multipath propagation, simple equalization, a simple transceiver architecture and efficient use of the available bandwidth through overlapping subchannels. On the other hand, OFDM has several disadvantages such as spectral leakage due to high sidelobes, and high peak-to-average power ratio (PAPR).
The demand for higher data rates has been increasing significantly. Several techniques have been studied and proposed to meet this demand, such as overlaying small cells over macro cells to allow spectral reuse, opening new bands to wireless communication, and utilizing the bandwidth more efficiently by spectrum sharing via cognitive radio. Since wireless systems are evolving towards a “network of networks” architecture where many networks are expected to share the spectrum, spectrally agile waveforms with small out-of-band leakage are important. To that end, the adjacent channel interference created by the spectral leakage of OFDM makes this waveform unsuitable for these networks.
As an alternative to OFDM, filter bank multicarrier (FBMC) modulation schemes, specifically OFDM-Offset quadrature amplitude modulation (QAM), have recently taken interest. OFDM-OQAM is another MCM technique where data on each sub-carrier is shaped with an appropriately designed pulse so that sidelobes are lower. A real data symbol is transmitted in each subchannel and on each OFDM-OQAM symbol. Consecutive OFDM-OQAM symbols are staggered. Adjacent subchannels overlap to maximize the spectral efficiency, creating intercarrier interference (ICI); and consecutive OFDM-OQAM symbols interfere with each other due to the long pulse, creating intersymbol interference (ISI). In an ideal single path Additive White Gaussian Noise (AWGN) channel, perfect orthogonality may be achieved and ISI/intercarrier interference (ICI) may be cancelled. The OFDM-OQAM transmitter and receiver may be implemented in an efficient manner by using the polyphase filterbanks. Although OFDM-OQAM offers less spectral leakage, its implementation in practical systems poses several challenges due to its complexity, latency, and more complex channel estimation and equalization algorithms in doubly dispersive channels. Therefore, it is desirable to design an OFDM-like, but spectral contained waveform with improved out-of-band emission characteristics.
Therefore, there is a need for an advanced waveform for spectral agile systems that is capable of sharing opportunistically available and non-contiguous spectrum resources with other users. The characteristics of such a waveform should include low out-of-band emission (DOBE), low in-band distortion, low complexity, low latency, low PAPR, robustness to frequency and timing asynchronous, and robustness to power amplifier (PA) nonlinearity. The existing baseband waveforms in those systems possess very large DOBE, which may make it difficult for the existing baseband waveforms to be used in spectral agile systems.